1. Field of the Invention
The present invention relates to an absolute encoder and a method of obtaining an absolute position.
2. Description of the Related Art
Conventionally, an incremental encoder or absolute encoder has been used to measure a position and angle. Incremental encoders record gratings at a given period (pitch) in a scale or disk, optically or magnetically read the movement of the gratings, and obtain a grating movement count, thereby outputting relative position information. Some incremental encoders output an absolute position in combination with an origin detection mechanism, in addition to relative position information. Some known method interpolates phase information obtained by dividing unit grating movement into 1,000 or more sections and outputs the interpolated phase information in order to improve the resolution of position information. Another known method cancels manufacturing variations in gratings by simultaneously reading a plurality of gratings instead of reading gratings one by one, and averaging them. As described above, the incremental encoders can output high resolution position information with high accuracy by interpolating phase information, and reading and averaging gratings. However, the incremental encoders require an origin detection process to obtain absolute position information, as described above. It is, therefore, necessary to perform origin detection upon power-on, resulting in disadvantage from the viewpoint of usability (productivity). Also, the incremental encoders are inappropriate for applications which require security assurance.
On the other hand, absolute encoders can output absolute position information by reading a binary pattern such as an M-sequence code using a light-receiving element array. Japanese Patent Laid-Open No. 2012-37392 discloses an absolute encoder which irradiates a relatively movable scale with a light beam emitted by a point light source, and receives light (transmitted light or reflected light) via the scale by a light-receiving element array. The absolute encoder disclosed in Japanese Patent Laid-Open No. 2012-37392 receives periodic signals having a light amount distribution of 11 periods by 12 light-receiving elements, respectively. The absolute encoder then acquires absolute position information by reading the amplitude of each periodic signal, and acquires phase information (interpolation information) at as high resolution as an incremental encoder by obtaining the phase of each periodic signal.
The absolute encoder disclosed in Japanese Patent Laid-Open No. 2012-37392 can measure an absolute position at as high resolution as an incremental encoder. However, in the absolute encoder disclosed in Japanese Patent Laid-Open No. 2012-37392, the processing time is long since the periodic signals obtained by the light-receiving element array need to be normalized to perform interpolation processing.